Editing Cosmological principle (section)

== Definition ==
Astronomer William Keel explains:

<blockquote>The cosmological principle is usually stated formally as 'Viewed on a sufficiently large scale, the properties of the universe are the same for all observers.' This amounts to the strongly philosophical statement that the part of the universe which we can see is a fair sample, and that the same physical laws apply throughout. In essence, this in a sense says that the universe is knowable and is playing fair with scientists.<ref name="Keel">{{cite book |author=Keel |first=William C. |title=The Road to Galaxy Formation |publisher=Springer-Praxis |year=2007 |isbn=978-3-540-72534-3 |edition=2nd |page=2}}</ref></blockquote>

As [[Andrew Liddle]] puts it, "the cosmological principle [means that] the universe looks the same whoever and wherever you are."<ref name="Liddle">{{cite book |author=Liddle |first=Andrew |url=https://archive.org/details/introductiontomo00lidd_717 |title=An Introduction to Modern Cosmology |publisher=[[John Wiley & Sons]] |year=2003 |isbn=978-0-470-84835-7 |edition=2nd |page=[https://archive.org/details/introductiontomo00lidd_717/page/n18 2] |url-access=limited}}</ref>

The two testable structural consequences of the cosmological principle are [[Homogeneity (physics)|homogeneity]] and [[isotropy]]. Homogeneity – constant density
– means that the same observational evidence is available to observers at different locations in the universe. Isotropy – looking the same in all directions – means that the same observational evidence is available by looking in any direction in the universe. Isotropy implies homogeneity, but an homogeneous universe could be anisotropic.<ref name=Peacock-1998>{{Cite book |last=Peacock |first=J. A. |url=https://www.cambridge.org/core/product/identifier/9780511804533/type/book |title=Cosmological Physics |date=1998-12-28 |publisher=Cambridge University Press |isbn=978-0-521-41072-4 |edition=1 |doi=10.1017/cbo9780511804533}}</ref>{{rp|65}}